In many constructions, such as aircrafts, ships, boats, sports cars, wind mills, and golf clubs, it is highly desirable to achieve lightweight constructions having high strength and stiffness. Often, the choice of material for such constructions is some sort of composite material, such as fiber-reinforced plastics (FRP). FRP materials often have a relatively high strength-to-weight ratio and are relatively resistant to fatigue and corrosion.
Many types of plastics and types of fibers may be combined to create a FRP and material properties, such as strength and elasticity, depend on choice and combination of matrix and reinforcement material respectively.
A FRP material contains a matrix material, being the plastic, and a reinforcement material, being the fiber. Commonly used types of reinforcement fibers are glass fibers, aramid fibers and carbon fibers.
Glass fibers are relatively inexpensive fibers, and are commonly found in FRP products, such as hulls for leisure boats, for which a low price is of higher importance than a high strength-to-weight ratio.
Carbon fibers are more expensive than for example glass fibers but offer a higher strength-to-weight ratio when used in a FRP material.
Not only the choice and combination of matrix and reinforcement materials affect the material properties of the composite material. Experience in the art shows that also the method of production of the composite material, including for example combination of different reinforcement structures in different layers and use of special molds, vacuum systems and/or autoclaves, may affect the material properties of the composite material.
When producing a composite material not only is it desirable to achieve good material properties but also it is often desirable to achieve a good production economy.
One factor which may affect production economy is the capacity of the production in terms of throughput. A high throughput makes it possible to distribute the fixed production costs on a higher number of units. Another factor which affects production economy is the amount of fixed costs, such as cost for expensive special machinery (for example large ovens or autoclaves). Also, direct and indirect material costs and material waste affect the total production costs.
WO 01/41993 A2, discloses a method of producing a composite material. The described method is an attempt to achieve a composite material without using large and expensive autoclaves for removing voids in the material, and the method comprises the steps of assembling a preform from a suitable reinforcement material, in a mold; tackifying the preform with a tackifier in the mold; vacuum debulking the tackifier preform; double bagging the debulked preform with an inner bag and outer bag to control bag relaxation and to improve vacuum integrity; and infusing resin to the debulked preform using a vacuum-assisted resin transfer molding process.
However, the method disclosed in WO 01/41993 A2 has disadvantages, such as being complex to perform, and giving a relatively low strength-to-weight ratio.
As background art, the considerably older U.S. Pat. No. 4,385,957, claiming priority from a German patent application filed in 1979, should be mentioned as well. The inventors of U.S. Pat. No. 4,385,957 have recognized that particularly in connection with the impregnation of structural components having large surfaces, for example, rotor blades of helicopters and the like, or in connection with structural components having complicated shapes, it is difficult to properly control the resin flow during resin injection. This flow control problem is, according to the inventors of U.S. Pat. No. 4,385,957, due to the fact that the temperature of the resin at any particular location determines the viscosity of the resin and thus also its flowability. One way of solving the flow control problem is to use expensive and very complicated heated molds. Another way of solving the flow control problem is to use a method such as the one disclosed in U.S. Pat. No. 4,385,957, in which a resin is injected into a carbon fiber webbing which may comprise one or several layers of carbon fiber web. The required flowability of the resin and its curing is accomplished by a heat application directly through the carbon fiber webbing which is heated by passing an electrical current through the carbon fibers. Thus, the carbon fiber webbing operates directly as an electrical internal heating mat in the structural component.
Another object of U.S. Pat. No. 4,385,957 is to provide a method for manufacturing of fiber compound components or structures of any size and/or shape without the need for expensive heated molds and with reduced energy consumption as compared to using heated molds.
According to the disclosed method, electric current is fed through a carbon fiber webbing in order to generate a curing temperature of the resin. U.S. Pat. No. 4,385,957 also suggests controlling the strength of the electrical current so that it is different in different areas of the webbing whereby the intensity of the heating may be adapted to the particular area or configuration of the webbing. An unheated and electrically conducting mold is used for defining the shape of the FRP component and glass fiber webbing is used to insulate the carbon fiber webbing from the conductive mold.
Disadvantages of the method of U.S. Pat. No. 4,385,957 are that the method gives materials having low durability with regards to repeated stress and long term stress, and a relatively low strength-to-weight ratio.
Further background art is disclosed in EP 1724306 A1 and U.S. Pat. No. 4,486,494.